The present invention relates to a thermal barrier coating applied to the surface of a superalloy article e.g. a gas turbine engine turbine blade, and to a method of applying the thermal barrier coating. The invention particularly relates to ceramic thermal barrier coatings more particularly to stabilized zirconia thermal barrier coatings.
The constant demand for increased operating temperature in gas turbine engines was initially met by air cooling of the turbine blades and the development of superalloys from which to manufacture the turbine blades and turbine vanes, both of which extended their service lives. Further temperature increases necessitated the development of ceramic coating materials with which to insulate the turbine blades and turbine vanes from the heat contained in the gases discharged from the combustion chambers, again the operating lives of the turbine blades and turbine vanes was extended.
It is known in the prior art to apply these ceramic coating materials by the thermal, or plasma, spray process onto a suitable bond coating, for example a MCrAlY alloy bond coating, which has been applied to the metallic substrate.
It is also known in the prior art to apply these ceramic coating materials by the physical vapour deposition process onto a suitable bond coating which has an alumina interface layer, for example a MCrAlY alloy bond coating, which has been applied to the metallic substrate.
It is also known in the prior art to apply these ceramic coating materials by plasma spraying or by physical vapour deposition processes onto an oxide layer on the metallic substrate.
The ceramic thermal barrier coatings deposited by the physical vapour deposition technique have benefits over the ceramic thermal barrier coatings deposited by plasma spraying. The main benefit is improved thermal shock resistance due to the columnar structure of the ceramic thermal barrier coating produced by the physical vapour deposition process. Other benefits are improved erosion resistance and improved aerothermal performance.
However, despite these advantages, the ceramic thermal barrier coating deposited by the physical vapour deposition technique exhibits a thermal conductivity which is greater than that of a ceramic thermal barrier coating, of the same or similar composition, deposited by the plasma spray process. For example the thermal conductivity of a zirconia-8 wt % yttria ceramic thermal barrier coating deposited by the PVD process is 2.0 W/m/K and the thermal barrier conductivity for the same ceramic thermal barrier coating deposited by the plasma spray process is 0.8-1.0 W/m/K. If all other factors are the same for the two methods of deposition of the ceramic thermal barrier coating, the greater conductivity of the ceramic thermal barrier coating deposited by the PVD process means that a greater thickness of ceramic is required to achieve the equivalent insulating effect when compared to the ceramic thermal barrier coating deposited by the plasma spray process. This is an undesirable property because this necessitates a greater weight of ceramic thermal barrier coating on the metallic components of the gas turbine engine, and this is particularly undesirable for rotating components, e.g. turbine blades, because the additional weight may limit the temperature of operation due to a corresponding reduction in creep life of the metallic turbine blade.
Our European patent EP0628090 discloses one method of reducing the thermal conductivity of a ceramic thermal barrier coating deposited by physical vapour deposition, in which layers are produced in the columnar grains by depositing alternately by pure physical vapour deposition and by plasma assisted physical vapour deposition. The layers in the columnar grains increase the resistance to heat transfer through the ceramic thermal barrier coating.
It is known from European patent EP0166097 to provide a ceramic thermal barrier coating of zirconia with a first metallic oxide, yttria, to stabilize the zirconia and a second metallic oxide, ceria, to reduce the thermal conductivity of the ceramic thermal barrier coating. The cerium ion has an ionic radius different to the ionic radius of the zirconium ion and hence reduces phonon thermal conductivity.
It is also know that the addition of a second metallic oxide to zirconia stabilized with a first metallic oxide, yttria, reduces the phonon thermal conductivity if the second metallic ion has a valency different to the zirconium ion because of the appearance of extra vacancies in the zirconia lattice.